Electronic properties of graphene tuned by two-dimensional crystals

Abstract

The theme of graphene tuned by two-dimensional (2D) crystals is illustrated in four topics. Few-layer graphene such as bilayer graphene, trilayer graphene and tetralayer graphene can be regarded as one graphene sheet tuned by other graphene layers. Trilayer graphene and tetralayer graphene with Bernal stacking are studied as representatives of few-layer graphene systems. The electronic property of trilayer graphene is similar to a superposition of a single-layer graphene and a bilayer graphene. The Landau level crossing is observed in both quantum Hall effect and quantum capacitance spectrum. Numerical simulations based on tight-binding models explain the experimental results and quantitatively determines the band parameters. The band structure of tetralayer graphene consists of two bilayer-like subbands. The experimental study of tetralayer graphene reveals some discrepancies with respect to theoretical predictions. A tight-binding model based on surface relaxation in few-layer graphene is proposed to explain the anomalies. Vertical stacking of two-dimensional crystals can produce various van der Waals heterostructures. The graphene sheet tuned by layered molybdenum disulfide (MoS₂) is one example of van der Waals heterostructure. The basic electronic properties of MoS₂ are characterized. Then the quantum capacitance of the heterostructure is systematically studied. Midgap states have been discovered, which are likely from point defects in MoS₂. The energies of the midgap states are then extracted through modeling the quantum capacitance of single-layer graphene. The result of transport measurement of graphene on MoS₂ proves the existence of charge impurities at the graphene/MoS₂ interface and the screening effect of MoS₂ substrates. The realization of the Kondo effect in graphene is a hot topic recently. The magnetic impurities are essential to realize Kondo coupling in graphene. A cobalt-based 2D crystal is employed to serve as a magnetic substrate for graphene. Though the temperature-dependent transport measurement reveals a sign of Kondo effect in the Co-complex supported graphene, the transport mechanism for such a system remains an open question.

Date of Award	2015
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology